Electrochemical electrode internal half cell

ABSTRACT

An electrochemical electrode internal half cell in which the conductor is sealed within a plastic shrink tube by means of an inner layer of plastic having a melting temperature less than that of the shrink tube. A porous plug closes one end of the tube and the section of the tube between the plug and the seal is packed with a mixture of a metal and a sparingly soluble salt of the metal. Methods of making the internal half cell are described.

United States Patent 1 Jerrold-Jones 1 5] Feb. 19, 1974 [54]ELECTROCHEMICAL ELECTRODE 2,183,531 12/1939 Allison 204/195 F INTERNAL HL 2,977,293 3/1961 Ingold 2,846,386 8/1958 lngruber 204/195 F [75]Inventor: Paul Jerrold-Jones, Claremont,

Calif: FOREIGN PATENTS OR APPLICATIONS Assigneez Beckman lnstrumeus Inc.729,575 5/1955 Great Britain 204/195 F Fullerton Cahf' PrimaryExaminer-T. Tung [22] Filed: June 19, 1972 Attorney, Agent, or Firm-JohnG. Mesaros; Robert J 211 Appl. No.: 263,782

[52] U S Cl 204/195 F [57] ABSTRACT 51 Int. Cl. G0ln 27/30, GOln 27/32ag f i gfisg fg i f f i ifl [58] Field of Search. 204/1 T, 195 F, 195 M,195 G tube by means of an inner layer of plastic havmg a meltingtemperature less than that of the shrink tube. [56] References cued Aporous plug closes one end of the tube and the sec- UNITED STATESPATENTS tion of the tube between the plug and the seal is 3,705,08912/1972 Grubb 204/195 F acked with a mixture of a metal and a sparinglysolu- I-Iaddad F ble alt of the metal Methods of making the inernal3,562,130 2/1971 H0016 et al 204/195 M half cell are deSm-ibed-3,463,717 8/1969 Koopman et al. 204/195 F 3,354,069 11/1967Jerrold-Jones et al. 204/195 F 5 Claims, 6 Drawing Figures PAIENTEQ FEB9 I974 SHEET 1 OF 2 FIG. I

FIG. 2

FIG. 3

ELECTROCHEMICAL ELECTRODE INTERNAL HALF CELL BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates toelectrochemical electrodes and, more particularly, to an internal halfcell for electrochemical electrodes.

2. Description of the Prior Art This invention relates to an internalhalf cell component for the measurement of electromotive force whichincludes a metal and a salt of the metal that will produce anelectromotive force when inserted into a solution of the salt. In theapplication of this type of half cell, the salt is usually onlysparingly soluble and is in equilibrium with its saturated solution, thesolution forming a portion of the complete electrode system. The voltagegenerated by the internal half cell when inserted into the solution is afunction of the concentration of ions in the solution of the speciesformed by the salt.

This type of internal half cell is widely used in reference electrodesfor ion potential measurements. Such internal half cells are also usedin electrochemical electrode structures employing ion sensitive barriersand used as indicating electrodes for the measurement of the ionconcentrations of solution.

The internal half cell of the present invention is applicable to eitherreference or indicating electrodes. However, the invention will bedescribed herein with respect to only reference electrodes.

An electrochemical reference electrode utilized for ion potentialmeasurements ordinarily comprises a salt bridge tube containing a saltsolution in which the internal half cell is immersed. Electricalconnection between the salt solution and the sample or test solution ismade by liquid contact via a suitably formed aperture or passage in thetube, generally referred to as the liquid junction structure. In certaintypes of reference electrodes the internal half is in the form of aglass tube which is filled with a mixture of an electrochemically activemetal and a sparingly soluble salt of the metal. The most widely usedmaterials are silver and silver chloride mixtures and mercury andmercurous chloride mixtures, the latter normally being referred to as acalomel mixture. An aperture is provided in the glass tube to providecommunication between the aforesaid mixture and the solution containedin the salt bridge tube of the electrode. Electrical contact is providedto the mixture by means of a wire which is selaed in the glass tube. Inorder to provide an effective seal, the wire is normally formed ofplatinum which has a coefficient of thermal expansion very close to thatof glass and thus is scalable in glass. While such internal half cellsare certainly effective from a functional standpoint, they require arelatively high degree of skill to fabricate. Moreover, the glassinternal half cell must be annealed during the electrode productionprocess, thus adding to the cost of manufacture. The requirement for theconductor of the internal half cell being formed of platinum also addsto the cost of the structure. In addition, being formed of glass, theconventional internal half cell is somewhat fragile. Thus, what isdesired is an internal half cell for electrochemical electrodes whichovercomes the aforementioned disadvantages of the conventional glassinternal half cell.

SUMMARY OF THE INVENTION The principal object of the present inventionis to provide a relatively low cost and unbreakable internal half cellfor an electrochemical electrode and a method for making the same.

According to the principal aspect of the present invention, there isprovided an internal half cell for an electrochemical electrode which isformed of a plastic material and thus does not require the use of aplatinum conductor. The half cell is constructed'by placing a conductorcoaxially within a plastic shrink tube. A layer of the plastic having amelting temperature less than that of the shrink tube surrounds theconductor at a point spaced from one end of the tube. The tube is heatedto a temperature sufficient to soften said plastic layer and shrink thetube so that the conductor will become sealed to the shrink tube bymeans of the intermediate plastic layer. Preferably the sides of theshrink tube are squeezed during the sealing operation to assure that acomplete seal is made. The shrink tube is then filled with a suitablepacking material comprising a metal and a sparingly soluble salt of themetal and the end of the tube is then closed with a porous plug therebycompleting the structure of the internal half cell. The resultinginternal half cell, being formed of plastic, is flexible and thus morerugged than the conventional glass internal half cells of the prior art.Moreover, the internal half cell of the present invention is lessexpensive to manufacture since it requires less skill to fabricate, noannealing step is required, and the invention eliminates the need forusing a platinum wire as the conductor for contacting the packingmaterial within the half cell.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial longitudinalsectional view of an electrochemical reference electrode employing theinternal half cell of the present invention;

FIG. 2 is en enlarged longitudinal sectional veiw of the internal halfcell illustrated in FIG. 1;

FIG. 3 is a sectional view similar to FIG. Z except tha the half cell isrotated about its longitudinal axis; and

FIGS. 4-6 illustrate various stages of fabrication of the internal halfcell of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing indetail, there is shown in FIG. 1 an electrochemical reference electrode,generally designated 10, incorporating the internal half cell 12 of thepresent invention. The electrode comprises a tubular body 14 closed atits upper end by means of a cap 16. A liquid junction structure 18closes the lower end of the tube. An opening 20 is provided in the wallof the tube 14 for permitting filling of the interior of the tube with asuitable salt solution 22, typically saturated potassium chloride. Anelastomeric sleeve 24 surrounds the tube 14 for closing the opening 20.The internal half cell 12 is positioned in the tube 14 so as to beimmersed in the salt solution 22. The conductor for the internal halfcell 12, not shown in FIG. 1, is connected to the'conductor 26 of acable 28 which extends from the cap 16 of the electrode and is adaptedto be connected to a conventional high impedance voltmeter, such as a pHmeter (not shown).

The tube 14 of the electrode may be formed of any suitable nonconductivematerial which is not attacked by the salt solution 22 or samples whichare to be tested. Preferably the tube is formed of a plastic material,such as polyvinyl chloride, polytetrafluoroethylene (Teflon), nylon,polyethylene, or the like. The liquid junction structure 18 is in theform of a porous plug, such as a porous ceramic or porous plastic plug,which is cemented in the tube 14. Alternatively, the tube 14 may beconstructed of the plastic material disclosed in copending applicationof Neti et al. Ser. No. 65,898, filed Aug. 21, 1970, assigned to theassignee of the present application and the liquid junction structure 18may be formed by making the lower end of the tube in the form of a thinion permeable wall as taught in said copending application. Of coursethe tube 14 could also be made of glass in the conventional manner ifdesired.

Reference is now made to FIGS. 2 and 3 which illustrate the structure ofthe internal half cell 12 of the present invention. The half cell 12comprises a double wall cylindrical plastic tube 30 including an innerlayer 32 and an outer layer 34. The outer layer is plastic shrink tubingwhile the inner layer 32 is formed of a plastic material which has amelting temperature less than that of the shrink tubing layer 34. Theshrink tube layer 34 is characterized by contracting when heat isapplied thereto. Suitable shrinkable plastics are polyolefins,polypropylene and polyvinyl chloride. A suitable plastic for the innerlayer 32 is polyethylene. A suitable two layer shrink tube product 30suitable for practicing the present invention is sold under the tradename Thermofit SCL by Raychem Corporation of Menlo Park, Calif.

The conductor 36 for the internal half cell is preferably a wire whichis unreactive with the salt of the packing material for the half cell.Any noble metal would be suitable. As mentioned previously, the wire 36need not be formed of platinum, which is the most expensive of the noblemetals. When the packing material for the internal half cell is amercury-mercurous chloride mixture, the wire 36 may be suitably formedof palladium. When the packing material contains silver chloride, thewire 36 may be formed of silver and, if so, the silver particles may beeliminated from the packing mixture.

As best seen in FIG. 2, the wire 36 extends partway into the tube 30with one end of the wire extending beyond the end 38 of the tube so thatit will be exposed and thus may be suitably connectedby soldering orlike to the conductor 26 of the cable 28 during assembly of theelectrode 10. The wire 36 is formed with several bends at the region 40whereat the wall of the tube 30 is constricted to provide relativelyflat areas 42 and 44 as best seen in FIG. 3. Preferably the bends areformed in the wire 36 so as to lie in a single plane which is parallelto the surfaces 42 and 44 of the tube 30. As seen, a portion of theinner layer 32 of the tube 30 intermediate the opposite ends 38 and 46of the tube completely surrounds and is bonded to the wire in the region40 thereof to form a seal in the area indicated by reference numeral 47.A porous plug 48, formed of packed cotton, porous ceramic, or the like,closes the end 46 of the tube 30. A packing material 50 of anelectrochemically active metal and a sparingly soluble salt of themetal, such as any of the materials discussed hereinbefore, fills thespace in the tube 30 between the seal 47 and the porous plug 48.

Before mounting the internal half cell 12 in the electrode 10, it ischarged with electrolyte, preferably saturated potassium chloride. Thismay be accomplished by immersing the half cell in a container of boilingelectrolyte which will cause the electrolyte to fill the voids in thepacking material 50. Alternatively, the internal half cell may beimmersed in a container of electrolyte and a vacuum applied thereto toevacuate the voids in the packing material. Thereafter the vacuum isreleased and atmospheric pressure forces the electrolyte solution intothe internal half cell to fill the voids in the packing material.Thereafter, the exposed end of the wire 36 is connected by solder to theconductor 26 of cable 28 and then the internal half cell 12 is mountedin the salt bridge tube 14 by potting the same therein with a suitableresin (not shown).

, The bends 40 formed in the wire 36 are a desirable feature in order toprevent the wire from twisting or shifting longitudinally in the tube 30when the internal half cell 12 is being assembled intothc salt bridgetube. Otherwise, the seal formed between the plastic layer 32 and thewire 36 may be damaged thereby producing an electrolyte leak paththrough the sealing area 47. Moreover, if by chance a leak path doesexist anywhere between the plastic 32 and wire 36, the bent portion 40of the wire provides a relatively long path which would minimize thechance of electrolyte leakage across opposite sides of the seal 47.

The manner of making the internal half cell 12 will now be described.Initially the wire 36 is formed with the bends 40 therein. The wire isthen inserted through the end 38 of the two layer plastic tubing 30.Theplane v. in which the bends of the wire 36 lie extends perpendicularwith respect to the sheet of drawing illustrating FIG. 4. Heat is thenapplied to the tube 30 at the location indicated by the arrows in FIG. 4which causes the outer shrink tube or layer 34 to contract and the innerlayer 32 to soften until the inner layer becomes sealed to the wire 36over the entire region of the bends 40 in the wire. Preferably presureis also applied to the tube 30 in the direction indicated by the arrowsillustrated in FIG. 4 simultaneously with the application of the heatthereto. After cooling, the assembly takes the form illustrated in FIG.5 of the drawing. The mixture of metal and sparingly soluble salt of themetal 50 is then packed in the lefthand end 46 of the tube 30 and theporous plug 48 is positioned-in the tube with its outer face 52 spacedslightly from the end 46 of the. tube. The region of the tube betweenthe seal 47 and the end 46 is then heated to contract the outer layer 34and compress the packing material 50 and plug 48 within thn tube. Theend 46 of the tube is then rolled over a heated surface to constrict thesame as shown in FIGS. 2 and 3 whereby the plug 50 will be firmlyretained in the assembly.

The internal half cell 12 can also be constructed by initially coatingthe bent portion 40 of the conductor 36 with a suitable relatively lowmelting temperature plastic such as polyethylene. Thereafter the coatedconductor is positioned in a plastic shrink tube formed of the samematerial as the layer 34 of the tube 30. Thereafter heat and pressureare applied to the shrink tube intermediate its ends in the same manneras describedin connection with-the assembly shown in FIG. 4 whereby thepolyethylene layer will soften and the shrink tube will contract andbecome sealed to such layer thus forming an assembly as illustrated inFIG. 5. The construction of the internal half cell is then completd inthe manner described above.

It will be appreciated that by the present invention there is providedan inexpensive and simple method for forming a flexible and unbreakableinternal half cell for an electrochemical electrode. The internal halfcell is less expensive than conventional glass internal half cells inthat it does not require the use of a platinum conductor which issealable to glass. Moreover the method of the presentinvention requiresless tooling and less mechanical skill than is required to produce aglass internal half cell.

It will be understood that various changes can be made in the form,details, arrangement and proportions of the various parts and in thematerials and steps utilized in the method of the present inventionwithout departing from the spirit and scope of the invention as definedby the appended claims.

What is claimed is:

I. An internal half cell for an electrochemical electrode comprising:

a plastic shrink tube;

a conductor positioned lengthwise within said tube,

said conductor being formed with a plurality of bends in a portionthereof;

a layer of plastic inside said tube forming a seal between said tube andsaid portion of said conductor at a point spaced from at least one endof said tube, said layer having a melting temperature less than that ofsaid tube, said tube and said layer being flattened at said seal;

a porous plug closing said one end of said tube; and

the section of said tube between said seal and said plug containing ametal and a sparingly soluble salt of said metal.

2. An internal half cell as set forth in claim 1 wherein said layer ofplastic extends to the opposite ends of said shrink tube.

3. An internal half cell as set forth in claim 1 wherein said metal andsalt are in the form of an intimate mixture filling said section of saidtube.

4. An internal half cell as set forth in claim 3 wherein said metal ismercury, said salt is mercurous chloride and said conductor is formed ofpalladium.

5. An internal half cell as set forth'in claim 1 wherein said conductoris formed of a metal other than platinum and is unreactive with saidsalt.

2. An internal half cell as set forth in claim 1 wherein said layer ofplastic extends to the opposite ends of said shrink tube.
 3. An internalhalf cell as set forth in claim 1 wherein said metal and salt are in theform of an intimate mixture filling said section of said tube.
 4. Aninternal half cell as set forth in claim 3 wherein said metal ismercury, said salt is mercurous chloride and said conductor is formed ofpalladium.
 5. An internal half cell as set forth in claim 1 wherein saidconductor is formed of a metal other than platinum and is unreactivewith said salt.